A healthy cardiac rhythm not only consists of a heart that beats at the proper pace, but the muscular contractions of the four chambers of the heart must also be properly mediated such that they can contract in a coordinated fashion. The heart has specialized conduction pathways in both the atria and the ventricles that enable the rapid conduction of excitation (i.e. depolarization) throughout the myocardium. Normally, the sinoatrial node (“SA node”) initiates each heart-beat cycle by depolarizing so as to generate an action potential. This action potential propagates relatively quickly through the atria, which react by contracting, and then relatively slowly through the atrio-ventricular node (“AV node”). From the AV node, activation propagates rapidly through the His-Purkinje system to the ventricles, which also react by contracting. This natural propagation synchronizes the contractions of the muscle fibers of each chamber and synchronizes the contraction of each atrium or ventricle with the contralateral atrium or ventricle.
The rate at which the SA node depolarizes determines the rate at which the atria and ventricles contract and thus controls the heart rate. The pace at which the SA node depolarizes is regulated by the autonomic nervous system which can alter the heart rate so that the heart, for instance, beats at a faster rate during exercise and beats at a slower rate during rest. The above-described cycle of events holds true for a healthy heart and is termed normal sinus rhythm.
The heart, however, may have a disorder or disease that results in abnormal activation preempting sinus rhythm, and resulting in an irregular heartbeat, i.e. an arrhythmia. Individuals with cardiac ailments, and especially those at risk of sudden cardiac death (“SCD”), may suffer from an irregular pace and/or uncoordinated mechanical activity wherein the myocardial depolarization and contraction of the chambers do not occur simultaneously. Without the synchronization afforded by the normally functioning specialized conduction pathways or the proper pacing by the SA node, the heart's pumping efficiency is greatly diminished and can thus compromise a patient's cardiac output. Several different factors may lead to the development of an arrhythmia, including atherosclerosis, thrombosis, defects in electrogenesis and nerve impulse propagation, influences of the sympathetic and parasympathetic systems, ischemia (inadequate oxygen supply to the cells due to lack of blood flow), and/or poor vascular control.
Despite advances in techniques of resuscitation, cardiac arrest and related cardiac disorders such as those mentioned above are associated with significant rates of morbidity and mortality. Due to the increasing incidence of SCD, chronic heart failure, and other life threatening cardiac ailments, cardiac dysfunction remains a major public health problem, especially in developed countries. For example, it is estimated that between cases of 250,000 and 300,000 occur per year in the United States.
As patients age and/or exhibit habits that increase their risk of heart disease, certain heart ailments appear suddenly, while others develop slowly over a period of time. In the cases where it seems as if the heart ailment has suddenly appeared (e.g., SCD), it is often the case that cardiac episodes have previously occurred, yet, due to the absence of noticeable pain, have gone undetected. For example, an estimated half of the 3-4 million Americans that suffer from heart attacks per year, suffer from “silent” infarctions that are not felt by the patients. In any event, individuals that experience traumatic cardiac events tend to have suffered from a series of preceding cardiac difficulties that occurred over an extended period of time.
A certain degree of damage results with each detrimental episode that the heart undergoes. This damage may manifest itself through altering the cardiac structure, altering the contractile function of the heart, and/or damaging the heart's electrical system. When a patient exhibits damage to the electrical system of the heart in particular, severe issues may arise without the patient's detection. For example, in about 30% of chronic heart failure patients, the disease process compromises the myocardium's ability to contract, which thereby alters the conduction pathways through the heart. This conduction disturbance can cause a delay in the beginning of right or left ventricular systole and thereby induce asynchronous atrial and ventricular activation. Atrial fibrillation and malignant ventricular arrhythmia are two examples of such arrhythmias that may result from cardiac disease, both of which often prove deadly. Atrial fibrillation in particular is one of the most commonly encountered arrhythmias, and it is correlated with increased mortality and morbidity due to thromboembolic complications, especially with respect to undetected asymptomatic atrial fibrillation. Furthermore, alterations in ventricular contractility and ventricular volume are frequently followed by cardiac decompensation, leading to severe symptoms and the necessity for immediate hospitalization.
While arrhythmias are detectable on an electrocardiogram (“ECG”) and are often treatable, as previously noted, the patient may be wholly unaware of the occurrence. If left untreated over time, in some patients an arrhythmia can lead to clinical instability and an increased risk of death. Accordingly, it is common for patients with chronic heart failure or other similar cardiac diseases to exhibit minimal or no symptoms, followed by—what appears at least to the patient to be—a sudden, drastic cardiac event that either requires immediate hospitalization or results in death.
In patients at increased risk for cardiac difficulties, such as those patients with chronic heart failure, continuous monitoring of cardiovascular data is critical. For example, in patients with chronic heart failure, right ventricle hemodynamic monitoring, left ventricle wall motion, and ECGs each provide important clinical information with a favorable impact on outcome. However, conventional monitoring devices are typically bulky and not conducive to daily activities. As previously mentioned, one common monitoring system for monitoring a patient's cardiovascular data is an ECG. An ECG records electrical signals from the heart via a series of electrodes attached to the patient's chest. Typically ECG equipment is large and cumbersome, and is not suitable for a patient to use after release from the hospital. Further, ECG results are not easy to decipher and are typically reviewed by a healthcare practitioner who is qualified to translate the results.
Another type of monitoring system is a Holter monitor, or an ambulatory electrocardiography device. The Holter monitor comprises a portable memory device for recording cardiovascular data collected through multiple ECG leads attached to the patient's chest. The memory device can be worn on a belt or in a case on a strap worn across the patient's chest. Due to the size of the leads and the Holter monitor's required placement, it is recommended that individuals wearing the Holter device wear layers or bulky clothes to disclose the ECG leads attached to their chest. In addition, because of the requisite placement of the leads, individuals who desire to wear the Holter device discretely must wear shirts with a high neck so as to disclose the entirety of the electrodes.
Typically, the Holter monitor is applied to the patient for only 24 hours. Accordingly, the patient must report back to the physician's office periodically to return the equipment and deliver the memory device. Thereafter, the information collected must be reviewed, and the signals inspected to determine if any cardiovascular abnormality occurred within the previous 24 hours.
Accordingly, monitoring devices capable of continuously monitoring the cardiovascular data of a patient are cumbersome due to the nature of conventional monitoring equipment. Due to the growing number of patients exhibiting cardiac disorders, there is a need for a technique and system that allows for early detection and long term, unburdensome monitoring of heart related disorders. Furthermore, such novel techniques and equipment should be easy to understand and implement, universally adoptable, and have competitive advantages over conventional heart monitoring devices, such as ECGs and Holter monitors.